Magnetic random access memory (mram) with enhanced magnetic stiffness and method of making same

ABSTRACT

A spin toque transfer magnetic random access memory (STTMRAM) element and a method of manufacturing the same is disclosed having a free sub-layer structure with enhanced internal stiffness. A first free sub-layer is deposited, the first free sub-layer being made partially of boron (B), annealing is performed of the STTMRAM element at a first temperature after depositing the first free sub-layer to reduce the B content at an interface between the first free sub-layer and the barrier layer, the annealing causing a second free sub-layer to be formed on top of the first free sub-layer and being made partially of B, the amount of B of the second free sub-layer being greater than the amount of B in the first free sub-layer. Cooling down the STTMRAM element to a second temperature that is lower than the first temperature and depositing a third free sub-layer directly on top of the second free layer, with the third free sub-layer being made partially of boron (B), wherein the amount of B in the third sub-free layer is less than the amount of B in the second free sub-layer.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to magnetic memory elementshaving magnetic tunnel junctions (MTJ) and particularly to improving theinternal magnetic stiffness of the free layer of the MTJ to reducethreshold voltage/current during writing thereto.

2. Description of the Prior Art

Magnetic random access memory (MRAM) is rapidly gaining popularity asits use in replacing conventional memory is showing promise. Magnetictunnel junctions (MTJs), which are essentially the part of the MRAM thatstore information, include various layers that determine the magneticbehavior of the device. An exemplary MTJ uses spin torque transfer toeffectuate a change in the direction of magnetization of one or morefree layers in the MTJ. That is, writing bits of information is achievedby using a spin polarized current flowing through the MTJ, instead ofusing a magnetic field, to change states or program/write/erase/readbits.

In spin torque transfer (STT) MTJ designs, when electrons flow acrossthe MTJ stack in a direction that is perpendicular to the film plane orfrom the pinned (sometimes referred to as “reference” or “fixed”) layerto the free (or storage) layer, spin torque from electrons transmittedfrom the pinned layer to the free layer orientate the free layermagnetization in a direction that is parallel to that of the referenceor pinned layer. When electrons flow from the free layer to the pinnedlayer, spin torque from electrons that are reflected from the pinnedlayer back into the free layer orientate the free layer magnetization tobe anti-parallel relative to the magnetization of the pinned layer.Thus, controlling the electron (current) flow direction, direction ofmagnetization of the free layer magnetization is switched. Resistanceacross the MTJ stack changes between low and high states when the freelayer magnetization is parallel or anti-parallel relative to that of thepinned layer.

However, a problem that is consistently experienced and that preventsadvancement of the use of MTJs is the threshold voltage or current usedto switch the free layer magnetization. That is, such current andthreshold voltage requirements are currently too high to allow practicalapplications of the spin torque transfer based MTJ.

Existing MTJ designs include a free layer, generally made of acobolt-iron-boron (CoFeB) alloy, formed on top of a barrier layer, whichis typically formed on top of the pinned (or fixed) layer. Prior toannealing, the thickness of the CoFeB is typically approximately one toa few nanometers. After annealing, the free layer separates into twodistinct layers, one of such layers, closest or adjacent to the barrierlayer, is formed with CoFe whereas the other of such layers, farthestand removed from the barrier layer, is made of CoFeB with a rich boron(B) content. The thickness of these layers is typically approximately0.5 nm or less for the layer that is made of CoFe and the other one ofthe layers that has a rich B content has a thickness that is thethickness of the free layer minus the thickness of the layer that ismade of CoFe, or in the example provided above, 2 nm. Due to thisformation, when annealing is performed, such MTJs exhibit a considerablelow stiffness. Thus, during programming or writing to the MTJ, theamount of threshold voltage required to switch the MTJ is undesirablyhigh. For example, an MTJ such as that described above with the materialan increase in internal stiffness of approximately 0.8×10-6 erg/cm, inits CoFeB part of its free layer after annealing, with a normalizedswitching reducing approximately 16%, in large part due to the layer ofthe free layer that is farthest removed from the barrier layer, theCoFeB part of the free layer.

Thus, the need arises for enhanced internal stiffness of the free layerof a spin torque transfer-based MTJ, used in a magnetic memory element,to reduce the threshold voltage or current required to switch the freelayer magnetization of such MTJs.

SUMMARY OF THE INVENTION

Briefly, a spin toque transfer magnetic random access memory (STTMRAM)element and a method of manufacturing the same is disclosed having afree layer structure with enhanced internal stiffness. Some of the stepsof manufacturing include depositing a first free sub-layer on top of abarrier layer, the first free sub-layer being made partially of boron(B), annealing the STTMRAM element at a first temperature afterdepositing the first free sub-layer to reduce the B content at aninterface between the first free sub-layer and the barrier layer. Theannealing causes a second free sub-layer to be formed on top of thefirst free sub-layer, the second free sub-layer being made partially ofB, the amount of B of the second free sub-layer being greater than theamount of B in the first free sub-layer. Cooling down the STTMRAMelement to a second temperature that is lower than the first temperatureand depositing a third free sub-layer directly on top of the second freesub-layer, with the third free sub-layer being made partially of boron(B), wherein the amount of B in the third sub-free layer is less thanthe amount of B in the second sub-free layer or void of B.

These and other objects and advantages of the present invention will nodoubt become apparent to those skilled in the art after having read thefollowing detailed description of the various embodiments illustrated inthe several figures of the drawing.

IN THE DRAWINGS

FIG. 1 shows the process of enhancing the free layer of a STTMRAM 10,using a method of the present invention.

FIG. 2( a) shows the relevant portion of a STTMRAM element 50, inaccordance with an embodiment of the present invention.

FIG. 2( b) shows a graph 60, representing the switching characteristicsof the element 50 vs. the switching characteristics of a prior artSTTMRAM element, both having CFB layer exchange of 1×10⁻⁶ erg/cm, andfurther shows a graph 62, representing the switching characteristics ofthe element 50 vs. the switching characteristics of a prior art STTMRAMelement, both having CFB layer exchange of 0.2×10⁻⁶ erg/cm.

FIG. 3 shows a flow chart of the steps performed in manufacturing thevarious embodiments of the present invention, in accordance with amethod of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following description of the embodiments, reference is made tothe accompanying drawings that form a part hereof, and in which is shownby way of illustration of the specific embodiments in which theinvention may be practiced. It is to be understood that otherembodiments may be utilized because structural changes may be madewithout departing from the scope of the present invention. It should benoted that the figures discussed herein are not drawn to scale andthicknesses of lines are not indicative of actual sizes.

In an embodiment of the present invention, a spin toque transfermagnetic random access memory (STTMRAM) element and a method ofmanufacturing the same is disclosed. Relevant layers of the STTMRAMelement include a free layer structure, having enhanced internalstiffness, and made of sub-layers using an annealing process. In formingthe free layer structure, a first free layer is deposited on top of abarrier layer, the first free layer being made partially of boron (B),then, an annealing process is performed on the STTMRAM element at afirst temperature and after depositing the first free sub-layer toreduce the B-content of the deposited first free sub-layer, at aninterface between the first free sub-layer and the barrier layer. Thisannealing process causes the B in the first free sub-layer to migratefrom the interface between the first free sub-layer and the barrierlayer to the top of the first free sub-layer and away from the barrierlayer. The STTMRAM element is then cooled to a second temperature thatis lower than the first temperature and a second free sub-layer isdeposited directly on top of the first free sub-layer, with the secondfree sub-layer being made partially of B or void of B, wherein theamount of B in the second sub-free layer is less than the amount of B inthe first free layer.

In an alternative embodiment, after deposition of the second freesub-layer, another annealing process is performed using a thirdtemperature that is higher than the second temperature and lower thanthe first temperature.

FIG. 1 shows the process of enhancing the free layer of a spin torquetransfer magnetic random access memory (STTMRAM) element 10, using amethod of the present invention. The STTMRAM element 10 is showninitially (at the far left of FIG. 1) to include a bottom electrode 16on top of which is formed an anti-ferromagnetic (AFM) layer 18 on top ofwhich is shown formed a pinned layer 20 (sometimes referred to as“pinning layer”), on top of which is shown formed an exchange couplinglayer 22, on top of which is shown formed a reference layer 24 on top ofwhich is formed a barrier layer 26 on top of which is formed a(magnetic) free layer 28. The pinned layer 24, the barrier layer 26 andthe (magnetic) free layer 28 (in addition to other free layers directlyformed on top of the free layer 28, such as the free layer 30) arecollectively referred to as “magnetic tunnel junction (MTJ)”. Depositionof the layer 28 on top of the layer 26 forms a “magneto-resistivejunction”. It is noted that the layers 24, 28, 29 and 30 are each, attimes, referred to herein as a “magnetic layer”.

In alternative embodiments, the element 10 (and the element 50 shown inFIG. 2( a)) is a magneto-resistive (MR) element.

The layer 26 (also referred to as a “junction layer”), in someembodiments, is made of oxide and in other embodiments is metallic.

The pinned layer 20 and the exchange coupling layer 22 and the referencelayer 24, which is also commonly referred to as the reference layer(sometimes referred to as “pinned layer” or “fixed layer”), arecollectively referred to as a “synthetic-antiferromagnetic (SAF) layer”.

The layer 28 includes B as one of its material, as discussed below.

Upon annealing, as will be described in further detail below, theSTTMRAM element 10 takes on the structure shown in the middle of FIG. 1with the free layer 28 becoming a layer that is formed of sub-layers, aninterface layer 29, immediately abutting the barrier layer 26, and aboron-rich layer 30. The layer 29 in one embodiment of the presentinvention has an effective thickness that is less than 1 nm and furtherhas a lower boron (B) content than the free layer 28 and is positioneddirectly on top of the layer 26, effectively interfacing with thislayer. The layer 30, does not interface with the layer 26 and is in factpositioned above the layer 29 and accordingly removed from the layer 26.The layer 30 has a higher boron (B) content than the free layer 28.Subsequently, as shown on the far right side of FIG. 1, another freelayer, free layer 32 is deposited directly on top of the free layer 30.The layer 32, in one embodiment of the present invention has no Bcontent and in another embodiment, has a B content that is less Bcontent than that of the layer 30.

It is understood that layers typically formed on top of a free layer inSTTMRAM elements are formed on top of the free layer 32. An example ofsuch a layer is a top electrode.

The process shown and described relative to FIG. 1 serves to enhance theeffective within-film exchange strength of the free layer.

The steps discussed above, relative to FIG. 1, are now described infurther detail with exemplary materials disclosed regarding each of thelayers. The barrier layer 26, in an exemplary embodiment, is made ofmagnesium oxide (MgO). The free layer 28, in exemplary embodiments ismade, but not limited to, cobolt (Co), iron (Fe), nickel (Ni), boron(B), tantalum (Ta), titanium (Ti), copper (Cu), zirconium (Zr), chromium(Cr) and platinum (Pt). The free layer 28 may be formed as a uniformcomposition single layer or formed as a multi-layer structure where thelayer adjacent to and on top of the barrier layer 26 is a differentcomposition than the layer further away from the barrier layer 26. Oneexample of such multi-layer structure of the free layer 28 can be, butnot limited to, a thin CoFeBX layer, with X being any of Ni, B, Ta, Ti,Cu, Zr, Cr and Pt, that is in one example less than 1 nanometers (nm) inthickness, with its Fe content being approximately 60%. In anotherembodiment, such a layer has a thicker of more than 1 nano meters CoFeBYlayer with Fe content ≦40%, where X and Y can be any of Ni, B, Ta, Ti,Cu, Zr, Cr and Pt.

During the annealing step discussed above, the barrier layer 26 and thefree layer 28 are annealed and as such are referred to herein as the“annealed structure”. Annealing is performed under normal MTJ annealingcondition, which is annealing the structure under a first temperaturethat is approximately more than or equal to 200 degrees Celsius (° C.)and less than or equal to 500° C., and preferably 270° C.-350° C. for aperiod of a few minutes up to a few hours. During annealing, there mayalso be a first magnetic field applied to the annealed structure.

After the annealing step, the STTMRAM element 10 is cooled down to asecond temperature that is lower than the first temperature. Due to theannealing process, the barrier layer 26 forms a crystalline structureand the free layer 28 also forms crystalline structure at the interfacebetween the barrier layer 26 and the free layer 28 with B being pushedaway from the interface and forming the layer 29 in structure 12 of FIG.1, and another B rich layer 30 above layer 29.

The free layer 29 has lower B content, and the free layer 30 has ahigher B content, than that of the free layer 28. At the secondtemperature, the free layer 32 is deposited on top of the free layer 30.As exemplary materials, the free layer 32 is made of: Co, Fe, Ni, B, Ta,Ti, Cu, Zr, Cr or Pt. The free layer 32 typically has between 80%-100%content of any of the following materials: Co, Fe and Ni. Afterdeposition of the free layer 32, additional non-magnetic and magneticlayers that are required to make a functional the STTMRAM element 10 canbe further deposited on top of the free layer 32. The free layer 32 mayalso be a multi-layered free layer and in such example, is made of twofree layers. In fact, additional free layers, either in the form of asingle layer or in the form of multiple layers may be utilized with eachsuch layer formed on top of an adjacent free layer.

After completion of deposition of the rest of the STTMRAM element 10, asshown and discussed relative to FIG. 1, the entire STTMRAM element mayoptionally undergo another annealing step with a third temperature thatis higher than the second temperature but lower than the firsttemperature.

For a typical MgO-based MTJ, the free layer is composed of Co, Fe, B,and sometimes other materials, for example, Ti, Ta, Cr, Ni, is depositedadjacent to the barrier layer 26, which is usually amorphous afterdeposition. After a high temperature (>200° C.) anneal process of themethods described herein, the barrier layer 26 forms a latticestructure, which in turn helps form a thin layer of CoFe with a latticestructure matching that of the barrier layer to be formed at theinterface between the free layer and the barrier layer. The matchinglattice structures of the barrier layer and free layer produces a highmagneto-resistance signal through the barrier layer in a typicalMgO-based MTJ. However, the free layer 30 that is away from the freelayer-barrier layer interface (“upper free layer”) typically has less Coand Fe content than when initially deposited because B is depleted fromthe interface region and migrates to the upper free layer. With Bcontent increasing in the upper free layer after annealing, the upperfree layer is regarded more amorphous than after deposition, where CoFecrystalline nano-structures may exist in the upper free layer in uniformdistribution but random orientation. Neighboring CoFe crystals affecteach other less through crystalline exchange coupling, which requiresoverlapping of electron clouds from adjacent atoms, due to increasedspacing by B and random orientation. Rather, they affect each other morethrough magneto-static coupling, which is a much weaker energy term thanexchange energy. With weak exchange interaction between CoFe crystalswithin the upper free layer, dynamic switching process of the upper freelayer together with the interface layer 28 is more chaotic and lessuniform, where higher order spin-wave modes can be easily excited due toweak effective exchange energy within the free layer. The higher orderspin-wave mode is generally an energy relaxation path, which willadversely slow down the switching process of the free layer bytransferring the spin torque injected energy to other higher orderspin-wave modes not related to the switching process. Therefore, it isbeneficial if the effective internal exchange energy (or internalstiffness) of the free layer (or switching layer, including layer 29 andlayer 30) can be increased so that higher order spin wave mode can besuppressed and energy dissipation during switching process can bereduced.

In accordance with the various embodiments of the present invention,easier switching of the storage magnetic layer by spin torque currentwith reducing effective damping of the free layer is realized byincreasing the effective internal stiffness of the free layer, randommodes during switching are suppressed and coherent mode is enhanced.With more coherent behavior of the magnetization of the free layerduring switching process, spin torque efficiency is increased, effectivedamping is dropped and switching becomes easier. Any STTMRAM elementdesign with any shape is contemplated. This method is to achieve lowerswitching current in STTMRAM by quenching the additional damping effectfrom low internal stiffness of the free layer.

FIG. 2( a) shows the relevant portion of a STTMRAM element 50, inaccordance with an embodiment of the present invention. The STTMRAMelement 50 is analogous to the STTMRAM element 14 except that thebarrier layer and the free layer of the STTMRAM element 50 are shown anddescribed in more particular detail, as is discussed below.

The STTMRAM element 50 is shown to include a barrier layer 52, whichwould be formed on top of the layer 24 of the element 14, in addition toa free sub-layer 54, formed on top of the layer 52, a free sub-layer 56,formed on top of the sub-layer 54, and a free sub-layer 58, formed ontop of the sub-layer 56. The sub-layers 54-58 form the free layer of theSTTMRAM element 50.

As in FIG. 1, the element 50 may be a magneto-resistive (MR) element insome embodiments and the layer 52 may be a “junction layer” made ofoxide or is metallic. Deposition of the layer 54 on top of the layer 52forms a magneto-resistive junction.

Upon the completion of an annealing step, such as described relative toFIG. 1, the free layer of the STTMRAM element 50 is transformed intomultiple sub-layers, namely, an interface layer (also referred to hereinas “free sub-layer”) 54, the free sub-layer 56 and the free sub-layer58, as shown in FIG. 2( b). The free sub-layer 54 is made of a CoFecomposition having a thickness of approximately 0.5 nm, with B beingmostly depleted. The sub-layer 56 becomes rich in B and is directly ontop of the sub-layer 54 and is approximately 1.3 nm in thickness. Thesub-layer 58, which is formed directly on top of the sub-layer 56 ismade of CoFe and has an approximate thickness of 0.3 nm.

The element 50 achieves stronger free layer within-film exchangestrength with the benefit of the sub-layer 58 being capped on top of thesub-layer 56. This is at least in part realized by the notion that thetotal Mst, which is the saturation moment (Ms) times thickness, of thefree layer (including sub-layer 54, 56 and 58), after annealing, isadvantageously the same as those of prior art STTMRAM elements. With theMTJ shape of the element 50 not changing, effective anisotropy of thefree layer and thermal stability related to the effective anisotropyalso remain unchanged. However, because the sub-layer 58 has awithin-film exchange of Aex=2×10⁻⁶ erg/cm, the free layer's effectivewithin-film exchange is higher than that experienced by prior artSTTMRAM elements because the sub-layer 56, which is rich in B, iscoupled to the sub-layers 54 and 58, which are high exchange CoFe layerson either side of the sub-layer 56 and the high exchange CoFe layers,i.e. sub-layers 54 and 58, determine more of the dynamics of theswitching process.

FIG. 2( b) shows a graph 60, representing the switching characteristicsof the element 50 vs. the switching characteristics of a prior artSTTMRAM element, both having CFB layer exchange of 1×10⁻⁶ erg/cm, andfurther shows a graph 62, representing the switching characteristics ofthe element 50 vs. the switching characteristics of a prior art STTMRAMelement, both having CFB layer exchange of 0.2×10⁻⁶ erg/cm. The x-axisof each graph represents the normalized switching voltage and the y-axisshows the normalized MTJ resistance. The solid lines with circles priorart and the dashed lines with dots represent the element 50.

The curves of each graph 60 and 62 are the MTJ resistance afterapplication of a given voltage for 5 nanoseconds (ns) to try to switchthe MTJ from high resistance state to low resistance state. The stepfunction shape of each of the graphs 60 and 62 represents a switchingevent, where the voltage at which the switching event occurs is theswitching voltage threshold. By using the sub-layer 58, the switchingvoltage is reduced by 9% over prior art in the case of free layerAex=1×10⁻⁶ erg/cm, as shown by the graph 60, and 16% over prior art inthe case of free layer Aex=0.2×10⁻⁶ erg/cm, as shown by the graph 62.Thus, using the sub-layer 58, the lower the B-rich free layerwithin-film exchange, the larger the performance improvement realized byreduction of the switching voltage. In fact, the switching voltages asshown by the dashed curves in the graphs 60 and 62 show littledifference, indicating that when the sub-layer 58 is used, the sub-layer56's within-film exchange strength is not a critical parameter any more.

However, the structure whose behavior is exhibited as shown by the graph62 is not readily realizable with existing film processing technology.For example, if the sub-layer 58 of 0.3 nm thickness is depositeddirectly on top of the sub-layer 56 and annealing is performed on thefilm to form the sub-layer 54 of 0.5 nm in thickness and the B-richsub-layer 56 of 1.3 nm in thickness, the high temperature used to obtaina suitable barrier layer and CoFe crystalline structure will inevitablycause B migration into the sub-layer 58 causing it to be undesirablyamorphous-like and reducing the within-film exchange of the sub-layer58. Thus, the “glue” purpose by the CoFe-2 layer is defeated. Inaccordance with a method of the present invention, a two-step depositionprocess is disclosed to achieve the switching current density reductionas in the various embodiments thereof.

To take advantage of lower switching voltage threshold, the free layer'seffective within-film exchange needs to be increased. However, sinceCoFeB is required for crystalline forming of CoFe interface layer andself-alignment of CoFe lattice to that of the MgO after annealing, CoFeBlow exchange material is still required for high TMR ratio MgO MTJ.

FIG. 3 shows a flow chart of the steps performed in manufacturing thevarious STTMRAM elements of the present invention, in accordance with amethod of the present invention. Examples of such STTMRAM elements arethe elements 14 and 50.

During the process of manufacturing the STTMRAM element, various layersare deposited on top of one another as shown in FIG. 1 and the steps ofFIG. 3 pick up the formation of figures starting from deposition of thebarrier layer. The steps of flow chart 70 therefore show only thosesteps performed after the barrier layer has been deposited. At step 72,a free layer (FL1), such as the free layer 28 of FIG. 1 is deposited ontop of the deposited barrier layer. In FIG. 2( a), this layer is thesub-layer 54.

Next, at step 74, an annealing process is performed, using a firsttemperature, on the STTMRAM element, as formed thus far. In an exemplarymethod, the first temperature used at step 74 is approximately more thanor equal to 200 degrees Celsius (° C.) and less than or equal to 500° C.Accordingly, the deposited barrier layer and FL1 are annealed as well.Next, at step 76, the STTMRAM element is cooled down to a secondtemperature that is lower than the first temperature. At this point, twosub-layers are formed, such as the layers 29 and 30 in FIG. 1 or thesub-layers 54 and 56 in FIG. 2( a).

Next, at step 78, another free layer, FL2, is deposited over the FL1while the STTMRAM element is at the temperature of step 76. In FIG. 2(a), the FL2 is the sub-layer 58 and in FIG. 1, the FL2 is the layer 32.Next, at step 82, another annealing process is performed using a thirdtemperature that is higher than the second temperature but lower thanthe first temperature.

Optionally, additional non-magnetic and magnetic layers are deposited ontop of the FL2, as may be suited and optionally a magnetic field isapplied to the STTMRAM element thereby improving the magnetic propertyof the FL2.

Although the present invention has been described in terms of specificembodiments, it is anticipated that alterations and modificationsthereof will no doubt become apparent to those skilled in the art. It istherefore intended that the following claims be interpreted as coveringall such alterations and modification as fall within the true spirit andscope of the invention.

1. A method of manufacturing a spin toque transfer magnetic randomaccess memory (STTMRAM) element comprising: depositing a first freesub-layer on top of a barrier layer to form a magnetic tunnel junction(MTJ), the first free sub-layer being made partially of boron (B);annealing the STTMRAM element at a first temperature after depositingthe first free sub-layer to reduce the B content at an interface betweenthe first free sub-layer and the barrier layer, the annealing causing asecond free sub-layer to be formed on top of the first free sub-layer,the second free sub-layer being made partially of B, the amount of B inthe second free sub-layer being greater than the amount of B in thefirst free sub-layer, cooling down the STTMRAM element to a secondtemperature that is lower than the first temperature; and depositing athird free sub-layer directly on top of the second free layer, with thethird free sub-layer being made partially of boron (B), wherein theamount of B in the third sub-free layer is less than the amount of B inthe second free layer.
 2. A method of manufacturing, as recited in claim1, further including the step of second annealing after depositing thethird free sub-layer, using a third temperature that is lower than thefirst temperature and higher than the second temperature.
 3. A method ofmanufacturing, as recited in claim 2, further including the step ofdepositing additional layers on top of the third free sub-layer.
 4. Amethod of manufacturing, as recited in claim 1, wherein the first,second and third free sub-layers are each made of cobolt-iron (CoFe). 5.A method of manufacturing, as recited in claim 1, wherein the firsttemperature is approximately more than or equal to 200 degrees Celsius(° C.) and less than or equal to 500° C.
 6. A method of manufacturing,as recited in claim 1, wherein the first free sub-layer is made of Co,Fe, Ni, B, Ta, Ti, Cu, Zr, Cr or Pt.
 7. A method of manufacturing a spintoque transfer magnetic random access memory (STTMRAM) elementcomprising: depositing a first free sub-layer on top of a barrier layerto form a magnetic tunnel junction (MTJ), the first free sub-layer beingmade partially of boron (B); annealing the STTMRAM element at a firsttemperature after depositing the first free sub-layer to reduce the Bcontent at an interface between the first free sub-layer and the barrierlayer, the annealing causing a second free sub-layer to be formed on topof the first free sub-layer, the second free sub-layer being madepartially of B, the amount of B in the second free sub-layer beinggreater than the amount of B in the first free sub-layer, cooling downthe STTMRAM element to a second temperature that is lower than the firsttemperature; and depositing a third free sub-layer directly on top ofthe second free layer, with the third free sub-layer being substantiallyvoid of B.
 8. A method of manufacturing, as recited in claim 7, furtherincluding the step of second annealing after depositing the third freesub-layer, using a third temperature that is lower than the firsttemperature and higher than the second temperature.
 9. A method ofmanufacturing, as recited in claim 8, further including the step ofdepositing additional layers on top of the third free sub-layer.
 10. Amethod of manufacturing a magneto-resistive (MR) element comprising:depositing a junction layer on top of a first magnetic layer; depositinga second magnetic layer on top of the junction layer to form amagneto-resistive junction; annealing the MR element at a firsttemperature after depositing the second magnetic layer; cooling down theMR element to a second temperature that is lower than the firsttemperature; and depositing a third magnetic layer directly on top ofthe second magnetic layer.
 11. A method of manufacturing, as recited inclaim 10, wherein the junction layer is made of oxide.
 12. A method ofmanufacturing, as recited in claim 10, wherein the junction layer ismetallic.